High-speed electronic digital-to-analogue converter system



Feb. 28, 1956 HIGH-SPEED ELECTRONIC DIGITAL-TO-ANALOGUE CONVERTER SYSTEM Filed April 2, 195s H. R. KAISER ETAL 2 Sheets-Sheet l Irfan/zx Feb 28, 1956 H. R. KAISER ETA. 2,736,889

HIGH-SPEED ELECTRONIC DIGITAL-TO-ANALOGUE CONVERTER SYSTEM ,/ffaiA/i y.

United States Patent O HIGH-SPEED ELECTRNIC DIGITAL-TO- ANALGUE CONVERTER SYSTEM Harold R. Kaiser, Woodlands Hills, Claude A. Lane, Culver City, and Wiiford S. Shociiency, K Torrance, Calif., assignors, by mesne assignments, to Hughes Alrcraft Company, a corporation of Delaware Application April 2, 1953, Serial No. 346,393

13 Claims. (Cl. 340-347) vrl`he present invention relates to digital-tojanalogue converter'systems and, more particularly, to alhigh-speed electronic digital-to-analogue converter system for vconverting a plurality of applied digital signal sets vto corresponding analogue output signals, the conversion being lperformed by changing the level of each analogue output signal until it becomes equal to the analogue equivalent Vof the corresponding applied digital signalset.

yDigital-to-analogue convertersof the general type provided by the present invention are essential components of any syst-ern wherein digital signals `are utilized tocontrol the operation of various electrical and mechanical devices. ln a particular situation, for example, theconverter of the` resent invention may be utilized in combination with a digital computer for translating the digital output signals produced by the computer into analogue signals which may be utilized to control such .analogue devices as synchros, servos, and the like.

Several types of electronic digital-toanalogue converters have been utilized in prior art systems. yInoue type of converter the entire range of digital signal sets and corresponding analogue equivalents are continuously produced and compared with the digital signal set to be converted. When the digital signal set is found which corresponds to the set to be converted, the corresponding analogue signal is gated to an output circuit for utilization. A system of this general type is described in.U..S. Patent Serial No. 2,533,242, entitled Data Transformation System by D. H. Gridley, issued December l2, i950.

While the Gridley system may be adapted to convert a plurality of applied digital signal sets to corresponding analogue signals on a time-sharing basis, it is apparent that the speed of operation is greatly limited since v.it is necessary to compare the entire range of digital signal sets with each digital set to be converted. .In addition, 50

vcomprising a plurality of flip-flops, one for each digit of the number. The dip-flops are utilized to control a decoding network which produces an `analogue output signal corresponding to the setting of theregister. One system of this type is described on pages V33-l1 to 33-:13 and in Fig. 7 of the article entitled Continuous Variable kinput and Output Devices by I. P. Eckert,:Jr.:in vol.

lil of Theory and Techniques for Design .of Electronic vDigital Computers, published June 30, 1948 by the Moore School of Electrical Engineering, Universitymf 35 verter v systems.

2,736,889 Patented Feb. 2,8, 1956 responding plurality of constant current sources into an kattenuator network, one tlip-op controlling each corresponding source. The attenuator network weights the current sources in accordance with `the code established for setting the ip-lops and a linear summation of cur- 10 rents results which, acting across the output impedance of the network, produces an output signal which is the analogue equivalent of the register setting. An improved system of this general type, but which utilizes constant voltage sources rather than constant current sources, is

l5 described and claimed in copendingU. S. patentapplication, Serial No. 239,077, entitled Digital-to-AnalogConverter by Siegfried Hansen, led July `28, 1951. In addition, another type of current-weighting converter iS described in U. S. patent, Serial No. 2,610,295, entitled Pulse Code Modulation Communication System by R. L. Carbrey, issued September 9, 1952.

The disadvantage of current and voltage weighting converter systems of the above-described type isthat the accuracy of the system is limited tothe accuracy of the constant voltage sources. As aresult, it is not possible t0 provide a highly accurate system of this type wherethe analogue output signal, and consequently the load conditions, must vary over a considerable range. While the voltage or current supplies maybe well regulated at some s points within therange they cannot be ymadeto-remain absolutely` constant throughout the range.

' The present invention discloses a highspeed electronic digtal-to-analogue converter system which `overcomes the` above and` other disadvantages of the prior art con- According to `the present invention the set of digital signals to be converted is initiallyentered into avip-iiop register, the setting of lwhich is continuously translated into an equivalent analogue reference signal bymeans of a novel decoding circuit-whichnrequires 40 neither constant current nor constant voltage sources.

The reference signal is applied to a comparator circuit which compares the reference signal with a variable output signal produced in an analogue .output circuit. The comparator produces signals Co and C o; respectively, indicating vthe positive andnegative sense ofthe diiference between the reference signal and the variable output signal. Signals Coand C0 are then utilized to control the analogue voutput vcircuit so that the variable output signal -is Achanged until it becomes equal to the reference signal andthus becomes the analogue equivalent of the applied set of digitalsignals.

The novel decoding circuit of the present invention includes a current switch for each ofthe digits of the input signal set. Each current switch-is controlled by a signal of an associated flip-hop of the register so that it produces a binary-weighted current only when the flip-Hop signal is in a high-level state representing binary l. It is not necessary that the ip-,op signals be very accurately regulated, as in the above-mentioned voltage and current weighting converter systemsLsince the c ur- *rent switches may be opened and closed by signals rwhich have a considerable range within which they may be effective.

Another factor which adds Yconsiderably tothe accuracy of the converter of the present invention is that theV decoding circuit need not produce an accurate reference signal throughout an entire range of load conditions. The Vreference signal need only be accurate when the dierence between the reference signaland the anxalogueoutput signal approaches` 0 volts. As a result, each of the current switches may be accurately designed to provide a binary-weighted current for the O-volt load conditions. in effect, then, the comparator circuit functions as a butter circuit between tie analogue load and the decoding circuit so that the voltage at the output of the decoding circuit is not influenced by a change in load conditions.

Where a plurality of digital sets are to be converted to corresponding analogue output signals an analogue output circuit is provided for each output signal and electronic switching circuits are introduced for selectively applying the output signals to the comparator circuit when the corresponding digital signal set is being converted. In one form, each of the analogue output circuits includes a storage capacitor which produces the above referred to variable output signal. Signals C and then are utilized, respectively, to control the charging and discharging of the capacitor so that the output signal is changed until it becomes equal to the reference signal.

Accordingly, it is an object of the present invention to provide an electronic digital-to-analogue converter system which may be utilized to control analogue devices over a wide range of analogue values and corresponding load conditions with a high degree of ac- A further object is to provide a decoding circuit forV translating an applied set of digital signals into a corresponding set of binary-weighted currents, the sum of which is an analogue output signal, each of the binaryweighted currents being produced or not produced in accordance with the open or closed condition ofV a corresponding current switch which is controlled by an associated digital signal.

'Still another object of the invention is to provide an electronic digital-to-analogue converter system wherein an applied set of digital signals are continuously translated into a reference analogue signal, the converter system including a comparator circuit which produces signals Co and C0, respectively, indicating the positive and negative sense of the difference between the reference gnal and the analogue output signal, signals C0 and C0 being utilized to control the changing of the level of the analogue signal until the level thereof becomes equal to the reference signal.

The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages thereof, will be better understood from the following description considered in connection with the accompanying drawings in which several embodiments of the invention are illustrated by way of examples. It is to be expressly understood, however, that the drawings are for the purpose of illustration and description only, and are not intended as a definition of the limits of the invention.

Fig. l is a block diagram of the basic embodiment of the digital-to-analogue converter of the present invention;

Fig. 2 is a schematic diagram of one form of the control circuit shown in Fig. l;

Fig. 3 is a schematic diagram of one form of the analogue output circuit shown in Fig. l;

Fig. 4a is a schematic diagram of one form of thef decoding circuit shown in Fig. l;

Fig. 4b is a schematic diagram of another form of the decoding circuit shown in Fig. l;

Fig. 5 is a schematic diagram of one form of the input switch shown in Fig. l; and

Fig. 6 signals appearing at various points in the embodiment of Fig. l during an illustrative digital-to-analogue conversion.

Referring now to Fig. l, there is shown one embodiment of a digital-to-analogue converter system according to the present invention. As shown in Fig. l the converter system comprises: a digital number register itl-il for receiving an applied set of digital input signals and producing corresponding output signals; a decoding circuit Zilli for translating the output signals from register lili) into a corresponding analogue reference sig nal; a plurality of analogue output circuits Sill), each producing an analogue output signal; a comparator circuit 599, responsive to the reference signal produced by circuit 209 and to an analogue output signal applied through one of a corresponding plurality input switches 400, for producing indicator signals C0 and Co respectively indicating the positive and negative sense of the difference between the reference signal and the analogue output signal; and a control circuit 604i responsive to signals Co and Co for selectively actuating input switches 400 to apply an analogue output signal to comparator lltl, and for actuating the corresponding analogue output circuit to change the level of its output signal until the output signal is equal to the reference signal, and thus is equal to the analogue equivalent of the applied set of digital signals.

illustrative forms of circuits suitable for control circuit ullil, analogue output circuits 3%, decoding circuit 200, and input switches 464i are shown in Figures 2, 3, 4a and 4b, and 5, respectively; and these circuits are considered in detail below. Digital number registers of the type required for register lll@ are well known in the cornputer art and may include a plurality of flip-flops or bistable multivibrators. Several suitable registers, for example, are described on pages 297 to 299 of Hig`n- Speed Computing Devices by Engineering Research Associates, published in i950 by McGraw-Hill Book Cornpany, Inc., New Yorlt and London. Suitable forms of comparator circuits suitable for use as comparator Sii() are illustrated on pages 335 through 353 of vol. 19 of the M. I. T. Radiation Laboratory Series, entitled Waveforms, published in 1949 by McGraw-Hill Book Company, Inc. ln the circuit shown in Fig. 9.44 on page 360 of Waveforrns, for example, the reference and the applied analogue signal may be signals e1 and e2, respectively; signals Co and C0 being then derived from anodes of the separate triodes of tube 6SL7.

It is convenient to consider the structure of control circuit 60G before considering the other circuits in detail because an understanding of control circuit edil provides an over-all picture of the digital-to-analogue converter system. Control circuit ehi) is mechanized according to logical equations, the equations being determine-:l through a logical consideration of the manner in which the conversion is to be performed; consequently the circuit is more readily understood by iirst considering the manner in which the conversion is to be performed, then deriving the defining logical equations, and linally considering the details of the specific circuits.

It has been explained that during the conversion opera* tion an analogue output circuit Euh is actuated by con trol circuit 6% to produce an analogue output signal which is continuously compared with reference signal representing the analogue value of the digital number to be converted. Where a plurality of digital signals sets are to be converted, each analogue output circuit includes a storage element for retaining a substantially constant is a composite diagram of the wave forms of switches dit() to comparator circuit 50G.

ifi-assess@ as the storage element, the storage capacitor being charged and discharged, respectively, when the reference signal is greater and less than the capacitor output signal. ln

f this case the capacitor Voutput signal, is the analogue output signal which is selectively `applied through one of n should be understood, however, that in lieu of capacitors many yother'types of stor'age'elements maybe utilized such as magnetic elements, cathode rayV tubes, or mechanical stepping devices.

In the above discussion, signals Co and C0, produced by comparator 500, arerconside'red as respectively indicating the positive and negative sense of the diierence vbetween the reference signal and an applied analogue output'signal. ln addition, signals `Co and may be de- 'flned as having levels representing binary 1 and 0, respectively, when the analogue output` signal is less'than the 'reference signal, and as having levels representing binary 0 and l, respectively, when the analogue output signal is greater than the reference signal. The latter definition is utilized throughout the discussion of the logical equations which follow. lt should be understood that the saine structure may be delinedas producing signals C0 and E? in either manner. l

During each digital-to-analogue conversion period, then, a capacitor is charged when signal Co` is l'and is discharged when signal 'l is l. It is possible, then, to

denne charging and discharging signals, designated hereinafter Ch and Dc, respectively, as'follows:

Ch=Co.Da

rwhere signal Da is produced by control `circuit 60()I and has a l-representing level during each digital-to-analogue conversion period.

In these equations the dot represents the logical and so that signal Ch assumes a l-representing level when signal C0 and signal Daare both in l-representing states. Similarly, signal Dc is l when signals Co and Da are l. l Y

When a ,plurality of capacitors are to becharged and discharged during different digital-to-analogue conversion V periods, it is necessary to add a switching signal tothe above-indicated algebraic charging and discharging definitions. Eor' this purpose an output switching signal Sak, produced by control circuit 699, is utilized; k being an 'integer representing the particular switching .s ignal.

Where 4 analogueoutput circuitsare utilized, for example, k is any of the integers l through fi. With the inl clusionof Signal Suk, the charging and'discharging functionsv then become: l i

AC'h=C0. Da.S0k Dc=C0.Da.Sok

and bok are l.

, One forni of circuit suitable for producingsignalsC/t and Dc is illustrated in Fig. 2 by a 'niatrix'650 which is included in control circuit oeil. For simplicity, matrix 650 is mechanized to'provide `.only'two sets of 'control signals 'for controlling the chargingof two capacitorsfaccording to digital input signals, itbeing understoodthat' a considerably greater number of capacitors' 'may be controlled in the saine manner. :Set (l) of the functions, appearing below, defines control`sig`nals Chl` and Dc1 which control the charging and discharginglof av first capacitor; and set (2) defines control signals and De2 controlling the charging and discharging of a second capacitor. Matrix 65h, then, is mechanized according to the following algebraic equations:

ieferring again to Fig. 2, lit will be noted thata signal generator 6i() produces signals Da, Da, S01, Sol, S02, and Soi', the waveforms of signals Da, S01 'and So.2v being illustrated in Fig, 6, where the other waveforms shown are those which occur during a'pa'rticular conversion operation which is described in 'detail` below. Signal generator 61) is not shown in detail, since such circuits are well known in the art. c

Each of the and functions of the equations of equation set (l) and set (2) above'isprovided by an and circuit in matrix 659. Thus, and functionCoDtLSol is provided by and circuit 651 having signals Co, Da and S01 applied to separate input terminals. In a similar manner each or function is provided by an or circuit such as or circuit 652 producing Vthe function or circuit 652 having signals C0, Da and S01 applied to separate input terminals.

@ne form of analogue outputcircuit for charging and discharging a storage capacitor 'under the control of signals Clt" and Dc is shown in Fig. 3, wherein it is noted that the output circuit comprises: a storage' capacitor 301; a cathode-follower output circuit 303; a charging circuit 310, including a triode/311 and a' diode 313; and a discharging circuit 320, including a triode 321 and ay diode 323.

The anode of triode v311 is connected to the anode of Vdiode'3l3 and is coupled through a loading resistor 314 to a source of positive potentiahnot shown. The grid and cathode oftriode v311"are coupled together through a resistor 315, thecathode being also connected to a source of negative potential, not shown. Signal l is applied to the'grid 'of triode 311 through a coupling capacitor 316 `and has a l-representing level such that triode 3411 conducts heavily when signal`11=1 and signal (271:0. Under these conditions, the anode voltage of triode 311 is suiiiciently negative to bias off diode-313 so that storage capacitor 301 cannot charge.

The (l-representing level of signal FCI-; is suliiciently negative so that the anode voltage of triode 311 is caused to rise above the highest charging potential of capacitor 3M. The negative level of signal 'C h may,rfor example, be 'low enough to cut ott triode 311. Thus, when signal Ch=0, and signal Ch=l, diode 313 isV caused' to conduct and capacitor 301 charges.

Triode 321, in discharging' circuit I320, has itsanode coupled through current-limiting resistor 322-to the cathode of diode 323, and through load resistor 324 to a source of positive potential, not shown. The grid of triode 321 is coupled through agrid resistor 325 to a lirst source of negative potential while its cathode is conlnected to a second source of ,negative potential, neither i of the vnegative potential sources being shown.` Signal Dc is applied to the grid of triode 321'through a coupling offenses capacitor 326. Signal Dc, and the signals produced by the sources applied to the anode, grid, and cathode of triode 321 are selected so that with signal Dc in a O-representing, or low-level state, triode 321 is cut off or only slightly conducting. As a result, the anode potential of triode 321 becomes sufficiently high so as to bias off diode 323, preventing the discharge of capacitor 301. The l-representing, or high-level state of signal Dc, is selected so that triode 321 is caused to conduct sufficiently to lower the anode potential thereof to ground potential; thus making it possible for capacitor 301 to discharge to zero potential. p

It is apparent, then, that the conditions Ch=1 (Ch=0) and Dc=1, result in the charging and discharging, respectively, of capacitor 301; and that capacitor 301 is neither charged nor discharged if both of signals Ch and Dc are 0. Thus, if it is not a digital-to-analogue period (Drz=), or if the particular output circuit is not in operation (S0lf=0), the corresponding capacitor is isolated from charging and discharging circuits and serves as an analogue memory. The period during which the signal produced by a capacitor reliably represents the desired analogue signal depends upon the leakage characteristic of the particular capacitor as well as the effectiveness of the diode switching circuits in the output circuit.

Consider now the operation of the system of Fig. l during a digital-to-analogue conversion, reference being made to Fig. 6 wherein the waveforms appearing at various points in the system of Fig. 1 during an illustrative conversion operation are shown. In the operation which is to be described, it will be assumed that the digital-toanalogue converter includes at least two analogue output circuits having first and second capacitors C1 and C2, corresponding to capacitor 301 in Fig. 3 for producing signals Cl and C2, respectively. Capacitors C1 and C2 are to be charged so that signals Cl and C2 finally represent the analogue equivalents of the binary numbers 1001 (9) and 1000 (8), respectively. It will also be assumed that signals Cl and C2 are initially at levels representing the analogue equivalents of the binary numbers 0101 and 1110 (14), respectively.

During the separate periods that the analogue output signals are developed across capacitors C1 and C2, signals Cl and C2 are applied through corresponding input switches to the input circuit of comparator 500, under the control of two input switching signals Sil and Sil, respectively, signals Sil and Sz'2 being produced by signal generator 610 in control circuit 600. The input switching signals are assumed, for the purpose of illustration, to be negative-going signals, since a negative signal is elfective to close an input switch of the type shown in Fig. 5, the circuit of Fig. 5 being described in detail below.

Referring now to Fig. 6, it will be noted that input switching signal Sil becomes negative prior to the time that output switching signal Sol rises to a 1representing level. As is explained in detail below, this is to allow sufficient time for the application of signal Cl (also shown in Fig. 6) through the corresponding input switch to the input circuit of comparator 500. During the initial period of operation, then, the signal appearing at the input circuit of comparator 500 assumes a level corresponding to the difference between the reference signal of decoding circuit 200 and signal Cl; comparator 500 responding to the sense of the difference signal and producing corresponding signals C0 and C0. in the particular operation which is illustrated signal Cl represents an analogue signal having a level (5 units) which is lower than the analogue equivalent of the binary input number (1001), and consequently the difference signal is positive and signals Co and Co have l and 0 representing levels, respectively.

It should be noted, at this point, that the initial period described above may be utilized to shift the binary number to be converted into register 100, although a parallel type of entry is equally suitable. Where the binary number is shifted in serially during the initial period of operation, the potential appearing at the input circuit of comparator 500 does not rise continuously, but will assume the final difference potential shown with substantially no delay after the binary number has been completely shifted into register 200.

In observing waveforms Da, Sol and Co of Fig. 6, it will be noted that at the time that signal Da first becomes 1, signals C0 and Sol are 1 so that the condition Da.Co.Sol :l is satisfied and, consequently, charging signal Chl becomes 1, or signals Chl =5+C0+S0l becomes 0. As a result, capacitor C1 is continuously charged until the difference potential appearing at the input circuit of comparator S00 assumes a negative sense and signals Co and C0 become 0 and l, respectively.

As soon as signal''is 1 the condition Dzz.C0.S0l- -1 is satisfied and signal Dc becomes 1. Capacitor C1 is then discharged until the difference potential again assumes a positive sense. Thereafter, capacitor C1 is alternately charged and discharged as the sense of the difference signal becomes positive and negative, respectively; the final value of signal C1 representing the analogue equivalent of binary 1001 within an error range which is less than the analogue equivalent of one-half of the least significant binary digit.

The amount that the analogue output signals may deviate from the desired conversion signal is determined by the hysteresis characteristic of comparator 500 and other delay or lag characteristics inherent in the switching circuits. The hysteresis characteristic of comparator 500 is due to the difference between its setting-to-l and setting-to-O input signal level. Thus, the difference signal appearing at the input circuit of comparator 500 must become positive by a predetermined amount before the comparator is set to 1, and must become negative by a predetermined amount before the comparator is set to 0. It is the hysteresis characteristic of comparator Stili, then, which causes the analogue output signal to alternately rise above and fall below the signal level corresponding to the exact analogue equivalent of the binary input number. In practical applications, however, it has been found that the hysteresis characteristic of comparator 500 does not influence the accuracy of the conversion system until it is attempted to accurately convert a considerable number of binary digits-as for example twelve or thirteen digits. The ultimate test as to whether the hysteresis characteristic is a limiting factor is whether or not the hysteresis range approaches the order of magnitude of the analogue equivalent of the least signicant binary digit.

It should now be apparent that the reference signal produced by decoding circuit 200 need not be perfectly accurate until the difference signal appearing at the input circuit of comparator 500 becomes of the order of magnitude of 0 volts. This feature alone makes it pos sible to achieve a higher accuracy than is possible in prior-art systems, even those utilizing a current-weighting type of decoding circuit of the type described above. With the improved decoding circuit of the present invention, however, an additional degree of accuracy is provided owing to the elimination of possible errors due to the variations of current or voltage sources. The reason for this will be more fully understood after the decoding circuits of the present invention shown in Figs. 4a and 4b are described in detail.

Referring now to Fig. 4a, decoding circuit 200 cornprises a plurality of current switches 2:10; n switches being shown corresponding to the n flip-flops in register 200, respectively. Each of the current switches 210-j (j being any of the integers l through n) has an input terminal 2li-j, an output terminal 212-5, and a control terminal 213-1'. A source of positive potential, not shown, is connected to input terminal 211-1 and output terminal 212-1' is coupled to ground through a first cur- "weight of the digit stored in ilip-iiop Rj.

ratsam rent-weightingnresistorr 214-1'. Signal Tij, produced by dip-flop Rj, is applied'to control' terminal 213-1' and is "dicating that the corresponding flip-hop registers a l. A When signal itil, and current switch ZIO-j is open, no

current passesy through either of current-weighting resistors 214-j or 21S-j; whereas when signal =0, current switch 210-j isrclo'sed and current flows through resi'storsy 214-1 and 21S-j.

`Resistors214-j and 21S-j are selected so that when l `the input conversion is completed the current through resistor 21S-j has ay value corresponding to the binary Since the voltage on output line 250 is volts at the end of the conversion period, the values'of resistors 21S-j are readily computed on the basis of a given source voltage and current switch impedance. The manner ofl computing the values of the current-Weighting resistors is fully explained after a specic form of current switch is considered in detail.

l In one form, each of current switches 210-j may be of i 'the type illustrated for current switch 21d-fi. As shown `resistor 219 to a source of positive potential, not shown,

the anode being also coupled to the cathode of a diode 220. The anode of diode 220 is'coupled through a resistor 221 to input terminal 211-11. The cathode of triode 216 is connected to a source of negative potential, not shown;

" the negative potential source being selected so that the conduction of 'triode 216 lowers the potential appearing at the anode of diode 22d to a negative value. rfhus ti'iode l 216 and itsv associate circuitry may be considered as a biasing voltage circuit responsive to high-level R signals `for producing a biasing voltage which is applied to diode 229. The anode of diode 220 is also connected to the anode of a second diode 222 which has its cathode connected to output terminal 212-11.

In operation, whenever signal is high and triode 216 is conducting the potential appearing at the anode of diodes 220 and 222 is suciently negative to bias off diode 222 so that it does not conduct. Thus, the current switch is open when signal T represents binary l, and Hip-flop Riz registers 0.

When signal TU is at a low level, indicating that flip-Hop Rn registers a 1, triode 216 is cut off, with the result that the anode potential thereof rises to a level which is high enough to bias oit diode 220. As a result, diode 222 is no longer biased off and conducts, allowing a binaryweighted current to ow through resistor 21S-n, which constitutes closure of the switch.

Since a binary-weighted current passes through each current switch 210-1 when it is closed underthe control 'of the associated flip-Hop signal lf=0, itis apparent A that the totalV current passing throughall ofthe current- Yweighting resistors'215 corresponds to the binary setting lofi; register 1260.2,Ak lt should also be apparent that'ithe suiu of these currents is present in lead 251i and is' the reference signal above referred te. Thus each of the current switches 210-j and its corresponding current-weighting resistors 214-j and 21S-j may be considered together as forming a current-weighting circuit. This current-weightling circuit isv responsive to a corresponding one of the digital inputsignals, represented by the output signal of the corresponding flip-flop '17d' of the digital number register 190 of Fig. l, for producing a unidirectional output current on common output lead 250. This output current corresponds in magnitude to the weight and value of the corresponding digital input signal.

` In one operative embodiment of the invention, register 208 includes a current switch of the type shown inFig. 4a, switch 21d-n; the potential values being those indicated in Fig. 4a, and the following circuit components being employed:

Triode 216 is one-halt of double triode 12AY7 Resistor 217-100K ohms Resistor 21S-120K ohms Resistor .2M- 150K ohms Diode ZZQi-one-half of tube type S726 Resistor 2li-10GB'. ohms Diode 222-other-half of tube 5726 With this type of current switch, the values of resistors 214-1' and 2155-1' are determined by assuming that the voltage drop across diode 2,22 when conducting is negligible and that the voltage at output line 250 is Ovvolts. The current which passes through resistors 21S-j, then must be weighted according to the corresponding binary place. Thus, if it is assumed that the conventional binary weighting is utilized and that resistor 21S-n is selected to 4pass one niilliampeie of current when switch 210n is'closed and line 250 is at 0 volts, resistor 215-(n-l), must pass one-half milliampeie of current under the same conditions, and so forth. Resistor 21S-rz is made to pass one milliarnpere of current by making both it and resistor 214-71 100K ohms, the parallel impedance thereof with respect to ground or 0 volt potential being 50K ohms. It is readily seen, then, that with +300 volts applied to resistor 221, two milliamperes of current passes through resistor 221 causing a voltage drop of 260 volts, and that one niilliampere or current passes through each of resistors 214-11 and 21S-1i.

For uniformity, each of the other resistors 21S-j may be 100K ohm resistors, resistors 214-1' being varied in lorder to provide the correct binary-weighted current through resistor 21S-j. Assuming that resistor 215-(n-l) is 100K ohms, the voltage drop thereacross must be 50 volts to provide a binary-weighted current is one-half milliampere. This means that the voltage drop across the corresponding resistor 221 must be 250 volts to make up the difference between 50 volts and the 300 volt supply.

, Since resistor 221 is 100K ohms, two and one-halt` milliamperes of current must pass through diode 220 when the switch is closed, and two niilliarnperes of current must pass through resistor 214-(11-1), since only one-half milliampere passes through resistor ZIS-(Iz-l). Resistor 2l4-(n-l)' must be 25K ohms since the voltage drop thereacross is 50 volts and the current through the resistor is two milliamperes. The lower place currentweighting resistors 214-j may then be computed in the sameV manner, resistor 214-(11-1), for example, being 10K ohms.

Another form of decoding circuit 200 is shown in Fig. 4b, wherein the binary-weighted currents are negative currents, and consequently the reference signal appearing on output line 256i has a negative amplitude corresponding to the setting of register 10i). The circuit coniyponents shown in Fig. 4b are designated by reference characters corresponding to the designation of like components in the decoding circuit of Fig. 4a, and theconnections in the circuit are the same as those in Fig. 4a

assess@ l l except that diodes 22@ and 222 have their electrodes connected in a reverse manner. All of the biasing voltages are the same, but the source voltage applied to input terminal 2li-j is -200 volts instead of +300 volts, as shown in Fig. 4a.

It will be noted that signals Rn, Rin-1), R1 are utilized to control the corresponding current switches so that when hiphop Riz, for example, is in its O state, triode 216 is cut olf and diode 21253 is forward biased raising the potential appearing at the cathode of diode 2253 sutliciently to cut oilE conduction through diode 222. When flip-liep Rn is in a l state triode 216 conducts, biasing o3 diode 220 and allowing current to pass through diode 220. The values of resistors 2idand 2id-j are computed in the same manner explained above.

The high-speed electronic operation of the digital-toanalogue converter of the present invention makes it practical to perform a considerable number of conversions on a time-sharing basis. A multiple conversion may be provided by including an analogue output circuit for each of the conversions desired and an input switch for selectively connecting each of the analogue output circuits to the comparator circuit. During the time that the energy in a storage element in an analogue output circuit is changed in the above-described manner, its signal is applied through a corresponding switch to the input circuit or", comparator Elli).

One type of input switch suitable for providing the high-speed electronic switching operation required in a multiple conversion system is illustrated in Fig. 5. Referring nos' to Fig. 5, it is noted that input switch 4th) includes first and second input terminals 401 and 402 for receiving the analogue signal to be converted and a control signal produced by control circuit 665i, respectively. Input terminal lll is connected to cathode of a first diode 463 having its anode coupled through a load resistor slid to a source of positive potential, not shown; the potential applied to resistor 4M, being greater than the full-scale level of the analogue input signal. The anode of diode d3 is also connected to the anode of a second diode 49S which has its cathode connected to an output terminal A o; output terminal d66 being connected to the input circuit ot comparator 5M). The junction do? of diode 433 and is connected to the anode of a triode llt'l'having its grid coupled through coupling capacitor 499 to input terminal 462, the grid of triode 40S being also coupled through a load resistor dit? to its cathode. The c thodc of triode is connected to a source of negative potential, not shown.

In operation, triode 53S is normally conducting so that junction ill' is held at a negative potential which biases diode 493 and so that they are nonconducting and, in effect, switch du? is open when a negative signal is applied to input terminal db2, triode 403 is cut oil, diode 493 becomes forward biased, and iunction 496 rises to a value which is substantially equal to the value of the analogue signal applied to input terminal 491, since the potential drop across diode 5.93 is negligible. The signal appearing at junction 496 then is transmitted through diode 46d, witl substantially no distortion, to the input circuit of comparator ddii.

Where a plurality of input switches of the type described above utilized, a. short period is allowed prior to each input conversion to insure that the output signal of the corresponding switch rises to the level of the associated analogue input signal. The rise of the switch output signal may, for example, be delayed due to shunt capacity across the parf connected switches. A set of typical waveforms illustrating the operation of two of a plurality of input switches, during two input conversions is shown in ig, o.

Waveform l-c', shown in Fig. 6, corresponds to signal Du discussed above except that it is periodic. Waveforms dil2-1 and db2-2 correspond to signals Sil and Sz`2 as previously described and represent the signals applied to input terminal 492 of two switches 4043-1 and `4110-2, respectively. lt will be noted that waveform 402-1 becomes negative, closing switch ddii-1, a short interval prior to the iirst high-level portion of signal Da', while Waveform 402-2 becomes negative a short time prior to the second high-level portion of signal Da. Each of signals 402-1 and 402-2 then remains negative throughout its corresponding conversion period.

Since the conversion system operates in the same manner where a pluralityy of divital signal sets are to be converted as where only one set is converted, it is not deemed necessary to consider the operation of a multiple conversion system in detail.

From the foregoing description it is apparent that the present invention provides a high-speed electronic digitalto-analogue converter system which may be utilized to control analogue devices over a wide range of load conditions with a high degree of accuracy. Part of the improvement in the accuracy is due to the novel arrangement whereby a comparator circuit functons as a buffer between the load and the current-weighting decoding circuit, and part ot the improvement is in the novel decoding circuit itself since the decoding circuit requires no well-regulated current or voltage sources.

While speciiic forms of circuits suitable for use as decoding circuit Zut), control circuit Still, analogue output circuits 3G@ and input switches 4% have been illustrated in detail it should be understood that the basic concept of the invention is not so limited. Decoding circuit Zilli, for example, may be replaced by a constant current or constant voltage type of decoding circuit, although with the disadvantages discussed above. In addition, analogue output circuits may be devised which operate directly in response to signals Ch and Dc without an inversion of signal Ck. Furthermore, there are many other arrangements suitable tor control circuit olli?.

What is claimed as new is:

l. An electronic converter for developing an analogue output signal corresponding to an applied set of N digital input signals by changing the level of a variable output signal until it becomes equal to the analogue equivalent of the digital input signals, said converter comprising: a register including N iiip-ilops for rcceivinU said digital input signals and producing N corresponding output signais; first means coupled to said register and responsive to said output signals for producing a reference signal having a level equal to the analogue equivalent of the N digital input signals in said register; second means for producing a variable analogue output signal, said second means including a storage capacitor for producing said variable output signal, and charging and discharging circuits coupled to said storage capacitor; third means coupled to said iirst means and to said second means and responsive to said reference signal and to said variable output signal for producing signals Co and @i when said reference signal is greater and less than said variable out put signal, respectively; and fourth means coupling said second and third means for actuating said second means to increase and decrease the level of the variable output signal in response to signals Co and C0, respectively, said fourth means including means for producing a charging signal C11 and a discharging signal Dc, said charging circuit being responsive to signal C11 to increase the level of said variable output signal in said capacitor and said discharging circuit being responsive to si; -al Dc to decrease the level ot said variable output signal.

2. The converter defined in claim l5 wherein said fourth means includes signal generating means for producing signals and Dc having one and zero-representing levels, respectively, during the formation of the analogue output signal and having zero and one-repre senting levels, respectively, at other times; wherein said fourth means also includes a logical and circuit and a logical or circuit coupled to said third means 13 andsaid signal generating means and vr'espt'nlsive to signals d, Da, and 'Ici for producing signals --znandmDc, said and circuit being responsive to signals and Da for producing signals De, and said or circuit being responsive to signals 5 and for producing signals'l; and wherein said second means also includes `r'rfeans responsive to signal for producing an ampliedsignal Ch representing` the complement ot signal El?, signals Ch and Dc then becoming eiective to cause saidcapacitor to increase and decrease the level of said outputsignal, respectively. v

, 3. A digital-to-analogue converter for converting a set A of digital input signals to a corresponding analogue out- .put signal, said converter comprising: a decoding'circuit responsive to the digital input signals for producing an analogue reference signal having a level corresponding to the set of digital input signals, said decoding circuit inpcluding a plurality of current switches, one foreach of the digital input signals, and a corresponding plurality yof sets of current-weighting resistors, eachy of said resistor sets being selected so that the amount of current which may pass through the corresponding current switch represents the analogue equivalents of the corresponding input signal, each of said current switches being operable in response tothe application of a l-representing digital input signal to allow the passage of a digitally-weighted amount of current through the corresponding resistor'set, the sum of all currents passing through said resistor' sets corresponding to said reference signal, each of said'current switches having a current-gating diode and a biasing diode, said biasing diode having a iirst electrode con- 7nected to the corresponding first electrode ofA said current-gating diode, said current-gating diodeA having'asecond electrode connected to the correspondingv set ofcur-` rent-weighting resistors, each of said current switches also including first means coupled to the second electrode of s aid biasing diode and operable through said biasing diode to prevent conduction through said current-gating diodeV when the corresponding digital signal is a O-representing signal, and including second means coupled to said current-gating diode for applying a current source to said current-gating diode, said current-gatingV diode Vthereby *passing current to said corresponding 'set' of current-` weighting resistors when said corresponding digital signal is a l-representing signal; an analogue output' circuit for y producing a variable output signal, said' output circuit output circuit to vary said variable 'output signal until it becomes equal to said reference signal, said control `vcircuit including means for increasing andY decreasing the level or" said variable output signal in response to said iirst and second levels of said two-level signal, respectively. Y

.4. in an electronic system for converting a set of digital ginput signals to an analogue output signal, a decoding circuit for producing a reference signal having a level equal to the analogue equivalent of the digital input signals, said decoding circuit comprising: a plurality of current switches, one for each of said digitalinput signals; a corresponding plurality of pairs of current-weighting resistors connected to said current switches, respectively; each of said current switches including irst and second Y unilateral devices, each device having anode and cathode electrodes, one electrode of said first device being connected to the same electrode of said second device in the 'corresponding current switch, each of said current switches also including control meansv responsive to the corresponding digital input signal 'for preventing conduction through said rst device, said control means being coupled to, and operable through, said second device; each of said pairs of resistors including first and second resistors, each resistor having one end connected to the other electrode of the corresponding first unilateral device, said resistors being selected so that said first resistor may pass a weighted current representing the analogue equivalent of said corresponding digital input signal, the sum of all currents through said rst resistors being thc reference signal; and means for applying a. current source to said one electrode of said first unilateral devices, said iirst devices passing a weighted current to the corresponding rst resistor when the corresponding digital input signal is a l-representing signal and said rst devices being non-conducting when said corresponding digital input signal is a O-representing signal and said control means is operable through the corresponding second unilateral device.

5. The decoding circuit defined in claim 9 wherein said oneV electrode of said devices is said anode electrode and said other electrode is said cathode electrode and wherein said control means includes a triode having an anode, a cathode and a control grid, the corresponding digital input signal being applied to said control grid, and the anode of said triode being connected to the cathode of said second unilateral device, said triode being operable in response to the O-representing level of the corresponding digital input signal to' forward bias said second unilateral device thereby effecting the application of conduction-preventing signal to said first unilateral device, and said triode being operable in response to the l-representing level of the corresponding digita] input signal to back bias said second unilateral device thereby effecting the application of a forward-biasing signal to said tirst unilateral device, allowing a positive current, corresponding to said digital input signal, to pass through said iirst unilateral device.

6. The decoding circuit detined in claim 9 wherein said one electrode of said devices is said cathode electrode and said other electrode is said anode electrode and wherein said control means includes a triode having an anode, a cathode and a control grid, the corresponding digital input signal being applied to said control grid, and the anode of said triode being connected to the anode of said second unilateral device, said triode being operable in'response to the O-representing level of the corresponding digital input signal to forward bias said second unilateral device thereby eecting the application of a conduction-preventing signal to said iirst unilateral device, and said triode being operable in response to the l-representing level of the corresponding digital input signal to back bias said second unilateral device thereby effecting the application of a forward-biasing signal to said iirst unilateral device, allowing a negative current to pass through said iirst unilateral device corresponding to said digital input signal.

7. A digital-to-analogue decoding circuit for translating an applied set of digital input signals into an output signal having a level equal to the analogue equivalent of thedigital input signals, said decoding circuit comprising: a plurality of current switches corresponding to said digital input signals, respectively; a corresponding plurality of current-weighting impedances connected to said current switches, respectively; and means for applying a current source to said current switches; said current switches being operable in response to the lrepresenting level of the corresponding digital input signal for electrically coupling said current sources applying means to the corresponding current-weighting impedance, said corresponding impedance thereby producing a weighted current representing the analogue equivalent of said corresponding digital input signal, the sum of all currents through said current-weighting impedances constituting said output signal; each of said current switches including a lirst unilateral device having one electrode connected to said current source applying means and the other electrode connected to said current-weighting impedance, and each of said current switches including a control circuit coupled to said rst unilateral device for preventing conduction through said first device when said corresponding digital input signal is a O-representing signal, said control circuit including a second unilateral device connected to said lirst unilateral device and a biasing circuit coupled to said second unilateral device for producing a conduction-preventing signal when said corresponding digital input signal is a O-representing signal, said conduction-preventing signal being applied through said second unilateral device to said lirst unilateral device, preventing conduction through said first device.

8. An electronic conversion system for forming M analogue output signals corresponding to M selectively a. plied sets of digital input signals, where M is an integer; said system comprising: first means for producing an analogue reference signal corresponding to the ap lied set of digital input signals; M second means for producing variable output signals, each of said second means being selectively operable for producing an analogue output signal corresponding to the applied set of d4 al input signals, and each of said second means including n storage capacitor for producing and storing said analogue output signal; third means cou4 led to said rst means and second means and responsive to said reference signal and to an applied output signal for producing comparison signals C0 and Ed when said reference signal is respectively greater and less than the applied output signal; fourth means coupled to said second and third means and operable in response to control signals for electrically applying said output signals to said third means during the time that the corresponding second means is made operable; iifth means coupling said second means and said third means for producing said control signalsA to selectively actuate said third means to increase and decrease the level of the corresponding variable output signal in response to signals C0 and EE, respectively, until said variable output signal constitutes said analogue output signal, said lifth means including signal generating means and matrix means, said signal generating means producing control signals Da and lit-zl having l and il-representing levels, respectively, during the formation or aid analogue output signals, and having (l and l-reprenting levels, respectively, at all other times, said signal generating means also producing M control signals Sek for acti ng corresponding ones of said second means where /r is an integer indicating the particular second neans `which is actuated, and said matrix means producing ivi pairs of control signal Chk and Dok for controlling the charging and discharging, respectively, of said capacitors, said matrix means including M logical and circuits and M logical or circuits, each of said and circuits being responsive to signals D@ and El?, and a correspond ig one of said M signals Sol for producing a corresponding one of signals De, and each of said logical o circuits being responsive to signals l and ilo, and a corresponding one of said lvl Signals for produc' corresponding one or" signals tk.

9. A digital-to-analogue converter for converting a set of N digital input signals to an analogue output signal, said converter comprising: decoding circuit means responsive to the N digital input signals for producing a corresponding reference signal having a level equal to the analogue equivalent of the digital input signals, said decoding circuit means including N corresponding current-Weighting circuits, each of said current-weighting circuits being responsive toa corresponding one of the N digital input signals for producing an output current, the sum of the output currents produced by said currentweighting circuits representing said reference signal, each of said current-weighting circuits having a current switch and a set of current-weighting resistors, each of said current switches including a gating diode coupled to said set of current-weighting resistors and responsive to a biasing signal for selectively gating ay current through said set of resistors, a biasing circuit responsive to said corresponding one digital input signal for producing said biasing signal, and a coupling diode connected to said biasing circuit and said gating diode for impressing said biasing signal on sad gating diode from said biasing circuit; an analogue output circuit for producing a variable output signal; a comparator circuit coupled to said decoding circuit means and to said analogue output circuit and responsive to said reference signal and to said variable output signal for producing control signals; and control means coupled to said analogue output circuit and said comparator circuit and responsive to said control signals for actuating said output circuit to vary the level of said variable output signal until said output signal has a level equal to that of said reference signal.

l0. An electronic digital-to-analogue converter for converting a set of digital input signals to a corresponding analogue output signal; said converter comprising: a decoding circuit responsive to the set of digital input signals for producing an analogue reference signal corresponding to the value of the set of digital input signals; an analogue output circuit for producing a variable analogue output signal, said analogue output circuit including an energy storage circuit for storing a charge therein and for developing said variable output signal having a level at any instant proportional to the charge in said storage circuit at that instant, and charging and discharging circuits coupled to said energy storage circuit and responsive to charging and discharging signals for respectively charging and discharging said storage circuit to increase and to decrease the level of said variable output signal; a comparator circuit coupled to said decoding circuit and said analogue output circuit and responsive to said reference signal and said variable analogue output signal for producing a rst and a second indicator signal when said reference signal is greater than and less than said variable analogue output signal; and a control crcuit coupled to said comparator circuit and said analogue output circuit and responsive to said indicator signals for producing said charging and discharging signals and impressing them on said analogue output circuit.

1l. An electronic converter for developing an analogue output signal corresponding to the total weight of N weighted digital input signals; said converter comprising: a decoding circuit responsive to the N digital input signals for producing a corresponding analogue reference signal, said decoding circuit including N current-weighting circuits and a common output, each or said currentweighting circuits being responsive to a corresponding one of the N digital input signals for developing an output current on said common output, the sumof the output currents produced by said N current-weighting circuits representing said analogue reference signal, each of said current-weighting circuits including a current switch and a set of resistors, said current switch being responsive to the corresponding digital input signal for gating an applied current to said resistor set, and said resistor set being connected between said current switch and said common output, each of said current switches including a biasing voltage circuit responsive to the corresponding digital input signals for producing a biasing voltage, a current-gating diode connected to said resistor set and responsive to said biasing voltage for selectievly gating said applied current to said resistor set, and a coupling diode connected between said biasing voltage circuit and said current-gating diode; an analogue output circuit including a storage capacitor for developing a variable output signal, and charging and discharging circuits coupled to said storage capacitor and responsive to charging and discharging signals for increasing and decreasing the level of said variable output signal; a comparator circuit coupled to said decoding circuit and said analogue output circuit and responsive to said reference signal and said variable output signal for comparing said reference signal with said output signal and for producing an indicator signal having a lirst and a second value representative of said reference signal being greater than and less than said analogue output signal; and a control circuit coupled to said comparator circuit and responsive to said indicator signal for producing said charging and said discharging signals, thereby to vary said variable output signal until said output signal is equal in magnitude to said reference signal whereupon said variable output signal constitutes the analogue output signal of the N weighted digital input signals.

l2. An electronic conversion system for forming k analogue output signals corresponding to k sequentially applied sets of digital input signals, where k is an integer, said system comprising: first means responsive to the k sequentially applied sets of digital input signals for sequentially producing k corresponding analogue reference signals; second means for producing the k analogue output signals in response to k control signals impressed thereon, said second means including k output circuits, each of said output circuits being selectively operable in response to a corresponding one of said k controls signals impressed thereon for producing a corresponding one of the k analogue output signals, each of said output circuits including a charging-discharging circuit and an energy storage circuit, said charging-discharging circuit being operable in response to the control signals impressed thereon for charging said storage circuit to store said one of the analogue output signals; third means coupled to said first and second means and responsive to said reference signals and said analogue output signals for producing comparison signals indicating equality or inequality between each of the k analogue output signals and its corresponding reference signal; and fourth means coupled to said second and third means and responsive to said comparison signals for producing said k control signals and impressing them on respective ones of said k output circuits.

13. A digital-to-analogue converter for converting a set of digital input signals to an analogue output signal, said converter comprising: an output circuit for directly generating an output signal; and means coupled to said output circuit and responsive to the set of digital input signals for varying the level of said output signal, said means including a decoding circuit responsive to the digital input signals for producing exclusively a corresponding reference signal having a level equal to the analogue equivalent of the digital input signals, a comparator circuit coupled to said decoding circuit and coupled directly to said output circuit and responsive directly to said reference signal and said output signal for producing comparison signals, and a control circuit coupled between said comparator circuit and said output circuit and responsive to said comparison signals for producing control signals and impressing them on said output circuit to vary the level of said output signal until the level of said output signal is equal to the level of said reference signal whereupon said output signal constitutes the analogue output signal.

References Cited in the tile of this patent The Binary Quantizer, Electrical Engineering. Das. 962-967, November 1949. 

